Light in a mid-infrared wavelength range (e.g., wavelengths of 5 to 30 μm) is an important wavelength range in the field of spectrometric analysis. As high-performance semiconductor light sources in this wavelength range, attention has been attracted to quantum cascade lasers (QCL) in recent years (e.g., cf. Patent Literatures 2 to 6, Non Patent Literatures 5 to 7).
A quantum cascade laser is a monopolar type laser element which uses a level structure including subbands formed in a semiconductor quantum well structure, to generate light by transitions of electrons between the subbands, wherein quantum well emission layers, each of which is formed in the quantum well structure and serves as an active region, are cascade-coupled in multiple stages, thereby enabling realization of high-efficiency and high-output operation. This cascade coupling of the quantum well emission layers is realized by use of electron injection layers for injecting electrons into emission upper levels, so as to alternately stack the quantum well emission layers and the injection layers.